In recent years, in association with enhancement of operation speed of personal computers, widespread use of network infrastructure, and realization of mass storage of data, as well as cost reduction of data storage, there are increasing occasions in which information of documents and images, having been conventionally provided in the form of paper printed matter, is received and viewed as more convenient electronic information.
As viewing methods for these items of electronic information, mainly utilized are conventional types such as liquid crystal display devices and CRTs. In recent years, light emitting types such as organic EL display devices have been used. Specifically, however, when electronic information is composed of pieces of document information, it is necessary to stare at these viewing devices for a relatively long time, which is certainly not viewer-friendly. It is commonly known that light emitting type display devices have disadvantages such as eye fatigue due to flicker, inconvenience of portability, limited reading posture, necessity to look directly at still images, and high power consumption due to the use of these devices for long-time reading.
As display devices to overcome these disadvantages, there are known memory-type reflective display devices, which utilize external light, resulting in consuming no electrical power to retain images. However, these devices do not exhibit adequate performance due to the following reasons.
Namely, a system, employing a polarizing plate such as a reflective type liquid crystal, creates a problem in white display due to its low reflectance of approximately 40%, and most production methods of constituent members are neither simple nor easy. Further, polymer dispersion type liquid crystals require high operating voltage and exhibit poor contrast of resultant images due to the image formation manner based on the refractive index difference between the used organic compounds. Still further, polymer network type liquid crystals result in problems such as high operating voltage and require complicated TFT circuits to enhance memory capability. Yet further, display elements using electrophoresis require a high operating voltage of at least 10 V and tend to exhibit low operation life due to electrophoretic particle aggregation.
In contrast, although being drivable at a low voltage of at most 3 V, electrochromic display elements have the disadvantage of exhibiting poor color quality (for black and the colors, namely yellow, magenta, cyan, blue, green and red), as well as having the disadvantage that each display cell requires a complicated film structure such as a vapor deposition film to ensure memory capability.
As a display system to overcome any of the problems in these electrochromic display systems, an electrochromic device, featuring a full-color display of a simple structure, bright and easily viewable, and low-power consumption, has been disclosed, wherein, for example, there is employed an electrochromic element, incorporating an electrolyte layer, being arranged between two conductive substrates, one of which is transparent, wherein the electrolyte layer contains an electrochromic dye which is colored and reversibly rendered colorless via at least one of oxidation and reduction (refer, for example, to Patent Document 1). Patent Document 1 discloses styryl dyes, leuco based dyes, viologens, and phenothiazines as specific examples of the electrochromic dye, but there are no descriptions or implications of imidazole leuco dyes.
Further, there has been disclosed a high resolution electrochromic element, incorporating a substrate and an electrochromic material applied to the substrate in a spatially resolved manner, wherein the electrochromic material exhibits a resolution of at least 75 dpi roughly (refer, for example, to Patent Document 2). Patent Document 2 discloses viologens containing phosphonoethyl groups as specific examples of the electrochromic dye, as well as a structure to incorporate porous electrodes as the electrochromic element, but there are no descriptions or implications of imidazole leuco dyes.
Still further, any of the electrochromic dyes described in Patent Documents 1 and 2 are radical cation type dyes, in which color is produced via reduction. The inventors of the present invention conducted detailed investigation and found that it was necessary to maintain adequate structural stability when employing these radical cation type dyes, resulting in an essential difficulty to ensure memory capability. In addition, it was found that a full-color image via these dyes, exhibited poor color image quality.
Patent Document 1: Japanese Patent Publication Open to Public Inspection No. 2004-151265
Patent Document 2: Japanese Translation of PCT International Application No. 2004-537743